1. Field of the Invention
The invention relates to apparatus and methods for performing automatic frequency control in communications receivers and, particularly for performing automatic frequency control using split-band signal strength measurements.
2. Description of the Prior Art
In general, a communications receiver receives transmissions of modulated signals and reproduces information carried by those signals. Typically, each modulated signal is allocated a defined channel having a center frequency and a bandwidth. The modulated signal within the channel has a center frequency and modulation bandwidth, i.e., the spectral width of the modulation. Under user control, the receiver is tuned to a channel from which the user wishes to receive information. In practice, such tuning involves initially tuning the receiver to receive a modulated signal centered at the channel center frequency. However, in many instances, the modulated signal is not precisely centered at the center frequency of the channel. If the modulated signal is not centered within the channel and the receiver contains an automatic frequency control circuit, the receiver adaptively retunes itself to compensate for the frequency offset of the modulated signal. Consequently, the modulated signal automatically becomes centered in the channel. As discussed below, circuitry for adaptively tuning the receiver is costly and complex.
In general, a conventional analog communications receiver for receiving and demodulating modulated signals contains a tuner, a demodulator and an automatic frequency control (AFC) circuit. Typically, the tuner is user controllable to enable individual channels to be selected for demodulation. After a channel is selected by the tuner, the demodulator demodulates the modulated signal within that channel. The demodulator also detects any frequency offset of the modulated signal center frequency from the channel center frequency. The AFC circuit compensates for such frequency offset. If the offset is not compensated, the demodulation process is not optimally accomplished. Consequently, any information thereby reproduced may be distorted.
The input lead of the tuner is typically connected to a transducer such as an antenna, photodiode, and the like, to convert electromagnetic energy into an electric signal. Alternatively, such a transducer may not be used and a cable transmission may be connected directly to the input lead of the tuner. For simplicity, I will assume, for the following discussion, that the receiver is utilized for over-the-air radio-frequency (RF) broadcasts such as conventional television or radio signals. Therefore, the transducer is an antenna and a signal received from the antenna is hereinafter referred to as a radio-frequency (RF) signal. Such an RF signal is defined by a center frequency and a modulation bandwidth, i.e., the spectral width of modulation carried by the RF signal.
Specifically, the tuner contains circuitry, connected to the antenna, for "down-converting" the RF signal from a relatively high center frequency to a lower center frequency. The circuitry utilized for performing the down-conversion is generally known in the art as an IF stage. Specifically, the IF stage contains a mixer, connected to the antenna, which "down-converts" the RF signal (lowers the center frequency of the RF signal) to an intermediate frequency (IF) thereby generating a so-called IF signal. After down-conversion, the modulation of the RF signal is centered at the intermediate frequency.
A mixer accomplishes down-conversion by mixing a local oscillator signal with the RF signal to produce the IF signal. To enable demodulation of information carried by different RF signals having different center frequencies, i.e., different channels, while maintaining a single center frequency for the IF signal, the frequency of the local oscillator is tunable. If more than one IF stage is used, typically, the first stage, i.e., the stage connected directly to the antenna, contains the tunable local oscillator.
More specifically, the RF signal from the antenna is "mixed" with a local oscillator (LO) signal to produce signals having center frequencies equal to the sum of the RF and LO frequencies, the difference of the RF and LO frequencies and various harmonics of the RF and LO frequencies. Typically, to achieve a low valued center frequency for the IF signal, an IF filter selects the difference signal as the IF signal to be demodulated. Initially, the LO frequency is tuned to enable an RF signal centered at the channel center frequency to be down-converted to an IF signal centered in the passband of the IF filter. However, if the center frequency of the RF signal is not equivalent to the center frequency of the channel, the IF signal will not be centered in the passband of the IF filter. Accordingly, the LO frequency is retuned by the AFC circuit to center the IF signal. In particular, to maintain a constant center frequency for the IF signal while the center frequency of the RF signal varies, the frequency of the LO signal varies with changes in the center frequency of the RF signal. In this manner, the difference frequency is maintained at the center frequency of the IF signal. As such, to maintain a constant center frequency for the IF signal, the frequency of the LO signal must be accurately and adaptively tuned during changes (drift) in the center frequency of the RF signal. The retuning of the LO frequency is accomplished by frequency control circuits. These circuits are typically contained within the demodulator.
A demodulator, connected to the IF filter, demodulates the IF signal to recover information from the modulation contained in this signal. In some instances, the receiver contains a second IF stage to further lower the frequency of the IF signal before demodulation. Additionally, as is well-known in the art, each IF stage may contain amplifiers, filters, limiters, and the like to facilitate the down-conversion. Because implementation of these circuits is conventional in the art and varies with each receiver application, I will not discuss them in any detail.
Conventional analog frequency control circuits, such as those used in conventional television or radio receivers, rely on the fact that a carrier (pilot tone) is present in the RF signal. Typically, the carrier is fixed relative to the modulation bandwidth. As such, the position (frequency) of the carrier within the IF passband serves as an indicator of the position of the IF signal within the IF passband. To use this indicator, the frequency control circuits search for the carrier within the IF signal. Typically, the search is accomplished by a conventional phase lock loop circuit. Once the carrier signal is found and locked upon, the demodulator also frequency locks to the carrier signal and proceeds to demodulate the IF signal.
In contrast to the foregoing description of a conventional analog receiver, digital communications systems, in general, do not broadcast a carrier signal along with the modulated RF signal. In fact, most digital communications systems intentionally suppress carrier signals in an effort to conserve spectral energy.
In general, digital communications receivers utilize the same circuitry as analog receivers. Specifically, digital receivers contain a tuner, a frequency control circuit and a demodulator. However, receivers for receiving digital broadcasts, such as high definition television (HDTV) broadcasts, typically include complex demodulator circuitry that demodulates information in the received signal and uses that information, or a portion thereof, to control the LO frequency of a local oscillator in the tuner. Generally, these circuits are known as carrier recovery circuits. Specific, well-known examples of such carrier recovery circuits include Costas loops, delay lock loops, and the like. Detrimentally, these circuits tend to accurately operate only when the center frequency of the received RF signal is within a relatively narrow frequency range, i.e., a frequency range known as a capture range. Specifically, capture range is the range of frequencies in which the center frequency of the RF signal must lie within in order to enable the demodulator to accurately control the frequency of the LO signal. Whenever the center frequency is outside the capture range, the demodulator can not accurately demodulate the IF signal. As a result, the carrier recovery circuit can not generate an accurate LO frequency control signal that will center the IF signal in the IF filter passband. Therefore, receivers for applications where the center frequency of RF signal may fall or drift outside of the capture range of the carrier recovery circuit contain further circuitry that maintains the center frequency of the IF signal within the capture range of the demodulator. Typically, such circuitry conducts orchestrated IF signal searches. These searches are performed by tuning the frequency of the LO signal in a logical pattern until the IF signal is frequency locked by the carrier recovery circuit. Such circuits contain search algorithms for performing the logical search. These search algorithms tend to incrementally scan the frequency of the LO signal until the carrier recovery circuitry achieves frequency lock, i.e., the center frequency of the IF signal is within the capture range of the carrier recovery circuit. Though search circuits and their concomitant search algorithms operate well in most receivers, these circuits add to receiver cost and complexity. Additionally, these circuits require relatively long periods of time to acquire lock of the IF signal. As such, during the acquisition time the receiver cannot demodulate the transmission and, consequently, a significant amount of information can be lost.
Therefore, a need exists in the art for an economical and simple technique that can quickly tune the frequency of an LO signal to a frequency which positions the center frequency of the IF signal within the capture range of a demodulator and which, to accurately tune the LO signal, does not require a carrier (pilot tone) in the RF signal.